A styrene-acrylonitrile (SAN) resin, which is a copolymer resin prepared by polymerizing styrene monomer (SM) as an aromatic hydrocarbon and acrylonitrile (AN) as an unsaturated nitrile, has been widely used in parts for an electrical/electronic product, a household product, an office supply, an automobile and the like due to its excellent transparency, chemical resistance, rigidity, and the like. Also, the SAN resin may be applied to an acrylonitrile-butadiene-styrene (ABS) resin to reinforce the low heat resistance of the ABS resin.
However, the SAN resin has a heat deflection temperature of about 100 to 105° C., and thus there is a limitation in applying the SAN resin to products requiring high heat resistance.
Accordingly, in order to impart excellent heat resistance to ABS, a method of introducing an α-methylstyrene (AMS) monomer is mainly used in the preparation of a SAN resin.
However, AMS has a low depolymerization temperature and may exhibit a degradation of heat resistance caused by a large amount of oligomers generated as a polymerization temperature increases. Therefore, unlike general SAN polymerization, a low polymerization temperature should be maintained, which results in a decrease in a polymerization rate.
In order to solve the problem of decreasing a polymerization rate as described above, a method of increasing a reaction retention time or introducing an excessive amount of an initiator has been used.
However, when a reaction retention time is increased, productivity may be degraded, a final product may exhibit a poor color, or a conversion ratio may be degraded. Also, when an excessive amount of an initiator is introduced, a defective molding may be caused by a decrease in weight average molecular weight.